1. Field of the Invention
The present invention relates to a solenoid type control valve used mainly on variable-capacity compressors of air conditioners for vehicles.
2. Description of the Prior Art
In FIG. 6 (Japanese Patent Preliminary Publication No. Showa 62-247186), a plurality of cylinders b and crank chambers c are formed in a compressor a. In the compressor, the cylinders are communicated through a valve plate e, which has a suction valve mechanism and a delivery valve mechanism, to a delivery chamber d and an annular suction chamber s.
In each cylinder b is installed a piston f, which is slidable therein and is connected through a piston rod i to a wobble plate h that is mounted on a drive shaft g driven by the engine. The delivery chamber d and the suction chamber s are connected to an external cooling circuit via a delivery port and a suction port.
As a pressure difference .DELTA.p (i.e. P.sub.c -P.sub.s) between the pressure P.sub.c in the crank chamber c and the pressure Ps in the suction chamber s increases, the stroke of the piston f becomes smaller, reducing the inclination angle of the wobble plate h and therefore the compressing capacity. Conversely, when the pressure difference .DELTA.p decreases, the stroke of the piston f increases, increasing the inclination angle of the wobble plate h and therefore the compressing capacity.
An air supply path p.sub.1 is formed to supply a high-pressure gas in the delivery chamber d to the crank chamber c. In the air supply path p.sub.1 is installed an external capacity control valve v.sub.1, which consists of a solenoid valve. To feed back, from the crank chamber c to the suction chamber s, gases bleeding from the compressing chamber in the cylinder b to the crank chamber c or gases supplied from the delivery chamber d to the crank chamber c through the air supply path p.sub.1, an air bleeder path p.sub.2 is formed between the crank chamber c and the suction chamber s. The air bleeder path p.sub.2 is adjusted in its opening degree by a self-capacity control valve v.sub.2.
When, during the startup of the compressor, the temperature in the vehicle to be cooled is high or the cooling load is high, the suction pressure P.sub.s is higher than a setting value P.sub.s0, causing the external capacity control valve v.sub.1 to be closed shutting off the air supply path p.sub.1. Since the crank chamber pressure P.sub.c is higher than the setting value P.sub.c0, the self-capacity control valve v.sub.2 is opened to open the air bleeder path p.sub.2. The pressure difference .DELTA.p between the crank chamber pressure P.sub.c and the suction pressure P.sub.s is kept smaller than the setting value .DELTA.p.sub.0. As a result, the piston f is reciprocated in the maximum stroke with the wobble plate h at a large inclination angle, performing the full-compressing capacity operation.
After this, as the compression operation proceeds, the suction pressure P.sub.s and the crank chamber pressure P.sub.c gradually decreases. When the crank chamber pressure P.sub.c reaches the setting value P.sub.c0, the self-capacity control valve v.sub.2 is closed to stop the gas flow from the crank chamber c to the suction chamber s.
As the temperature in the car lowers reducing the cooling load and the suction pressure P.sub.s becomes lower than the setting value P.sub.s0, the external capacity control valve v.sub.1 is opened, supplying the high-pressure gas from the delivery chamber d to the crank chamber c through the air supply path p.sub.1. This results in an increase in the crank chamber pressure P.sub.c, increasing the pressure difference .DELTA.p. This in turn reduces the stroke of the piston f, inclining the wobble plate h in a direction that reduces the compressing capacity.